The present invention relates generally to an image forming apparatus, and, more particularly, to an apparatus incorporating a photoreceptor belt having line indicia on the surface thereof whose location must be precisely detected for image registration purposes.
High speed copiers and digital printer machines typically employ a photoreceptor belt as the imaging media since the belt can provide significantly more area to form a plurality of latent images during a single pass or revolution when compared to a machine with a photoreceptor drum as the imaging surface. The position of the belt during operation must be known with a great deal of precision since the system timing control must ensure that the images are formed within preselected frames on the photoreceptor surface and exposure must be coordinated with the development, transfer and paper feeding functions. This is especially true for color printing machines. Positional errors occur because the belt is subject to changes in physical size and its rotational speed may vary because of mechanical wear and mechanical tolerances in the drive components. Various techniques have been developed in the prior art to compensate for these errors. These techniques include measurement of the movement of the belt in some fashion so that images are formed at the same relative location from revolution to revolution.
One technique uses holes formed in the belt at a predetermined distance outside of the image exposure frame. The passage of the holes through the viewing nip comprising a light source and a sensor generates an output signal which is used for registration purposes. Co-pending application, U.S. Ser. No. 07/807,927, filed Dec. 16, 1991, discloses this technique in a system using a Raster Output Scanner (ROS) as the imager. Co-pending application, U.S. Ser. No. 07/807,931, filed on Sep. 18, 1992, discloses the technique with LED print bars as the imager. Both of these applications are assigned to the same assignee as the present invention and are hereby incorporated by reference.
Formation of belt holes has disadvantages in that is reduces the structural integrity of the belt and requires an extra process step when forming the belt. One alternative method is to form registration marks on the surface of the belt, either at manufacture or by exposing the belt in a non-image area with a target mark and developing the mark with toner. The mark can then be sensed through the photoreceptor belt. The marker, rather than the hole, is then sensed. This technique requires a partially, light transmissive type of photoreceptor.
The following patents contain disclosure of techniques for sensing marks placed on the photoreceptor belt for various purposes.
U.S. Pat. No. 4,912,491 discloses an apparatus for forming superimposed images and registration marks corresponding to the position of the images associated therewith. The registration marks are formed apart from the imaging portion of the medium in a transparent area to be illuminated from the opposite side of the belt. Detectors sense the position of the registration marks as the marks pass between the illuminated areas. The sensing of the registration marks is used in determining proper registration positioning, whereby the image forming devices may be adjusted to achieve such registration.
U.S. Pat. No. 32,967 discloses a web tracking system for a continuous web which passes along a predetermined path through one or more processing stations. The tracking system has aligned tracking indicia on one or both sides of the web and detectors sense these indicia which are indicative of dimensional changes in width and length of the web at a particular point. An edge sensor is also provided to determine movement of the web.
Co-pending application, U.S. Ser. No. 07/930,642, filed Aug. 17, 1992, assigned to the same assignee as the present invention, discloses an apparatus and method of positional tracking of a moving photoconductive belt and adjusting an imager in an electrophotographic printing machine to correct for alignment errors when forming a composite image. Registration errors are sensed by developing an appropriate set of target marks, detecting the target marks, and controlling the position of the imager. The contents of this application are hereby incorporated by reference.
For these prior art, mark-detecting systems, a high precision of accuracy is needed when detecting the presence of the registration marks. This is particularly true for systems such as the referenced co-pending application where the composite color image registration requirements set forth therein (25 .mu.) become even more stringent. The present goal of an accuracy to .+-.5 .mu. has heretofore proved unattainable. Prior art detection systems that have been disclosed have merely been developed to generate output signals indicating the presence of a mark without trying to determine the center of the mark. This is especially true when the mark takes the form of a solid line. It will be appreciated that if the center of the detected line mark could be determined with a high degree of accuracy, the output signal could then be used to produce a very accurate registration and control signal. By sensing the center of the line, errors arising from both uniform and non-uniform line width variations, photoreceptor transmissivity, xerographic developed mass, contamination state of the sensor, flux intensity variations of the light source and any other similar errors are to first order eliminated.
According to one aspect of the present invention, there is provided an improved detection circuit for detecting line marks on the surface of a photoreceptor belt. In one embodiment, detection circuitry is used in conjunction with a photosensor on one side of the belt which senses the mark by use of a light source on the other side of the belt. More particularly, the present invention relates to an electrophotographic printing machine which incorporates a photoreceptor belt which moves along a preselected path,
imaging means for forming a plurality of registered color images on the surface of said belt, said color images formed in overlying registration to form a composite color image,
means for forming at least one target on the surface of said belt,
means for sensing said target and for generating an output signal representing detection of the center of said target, and
means responsive to said output signal for maintaining the overlying registration of said color images.